In order to meet wireless data traffic demands that have increased after 4th generation (4G) communication system commercialization, efforts to develop an improved 5th generation (5G) communication system or a pre-5G communication system have been made. For this reason, the 5G communication system or the pre-5G communication system is called a beyond 4G network communication system or a post long term evolution (LTE) system.
In order to achieve a high data transmission rate, an implementation of the 5G communication system in an mmWave band (for example, 60 GHz band) is being considered. In the 5G communication system, technologies, such as beamforming, massive MIMO, full dimensional MIMO (FD-MIMO), an array antenna, analog beam-forming, and a large scale antenna, are discussed to mitigate a propagation path loss in the mmWave band and to increase a propagation transmission distance.
Further, technologies, such as an evolved small cell, an advanced small cell, a cloud radio access network (cloud RAN), an ultra-dense network, device to device communication (D2D), a wireless backhaul, a moving network, cooperative communication, coordinated multi-points (CoMP), and interference cancellation, have been developed to improve the system network in the 5G communication system.
In addition, the 5G system has developed advanced coding modulation (ACM) schemes, such as hybrid frequency-shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC) and advanced access technologies, such as filter bank multi carrier (FBMC), non orthogonal multiple access (NOMA), and sparse code multiple access (SCMA).
Communication using a shared band has to comply with transmission regulations determined for a band to be used. These transmission regulations use various types of methods in order to mitigate signal interference between devices. Examples of the methods include a method of restricting transmission power to prevent reception power at a predetermined distance from exceeding a particular value, a method of hopping the location of a resource in a time or frequency domain, a method of using only some of all resources, and a method of receiving a signal from a different device first and making transmission possible when the reception power of the signal is lower than a particular value. A representative frequency band is an unlicensed band called a “License-exempt” or “Unlicensed” band. While a 5 GHz unlicensed band will hereinafter be described as an example in the present specification for the convenience of description, the following description may also be applied to other frequency bands based on similar shared regulations.
A communication device that uses an unlicensed band in a communication system may be divided into, for example, frame-based equipment (FBE) and load-based equipment (LBE). Each communication device satisfies the following regulations.
First, an FBE performs clear channel assessment (CCA) for 20 ms or more before a communication device performs transmission. The CCA may be understood as an operation of measuring the magnitude of interference before the communication device performs the transmission and determining whether another communication device uses the current unlicensed band. The FBE does not perform the transmission when the measurement result shows that the magnitude of the interference is greater than or equal to a predetermined value, and performs the transmission when the magnitude of the interference is less than the predetermined value.
FIG. 1A illustrates an example of a CCA operation of an FBE in an unlicensed band according to the related art.
Referring to FIG. 1A, if the FBE performs CCA 101 once, the FBE may occupy the unlicensed band for a time duration 103 of 1 ms to 10 ms and then has to rest without performing transmission for a time duration of at least 5% of the occupancy time 103. This is called an idle period 105. If the CCA execution result of the FBE shows that another communication device currently uses the unlicensed band, the FBE may perform CCA 109 again after a fixed frame period 107 passes.
Second, similar to the FBE, the LBE performs CCA for 20 ms or more before the LBE performs transmission. Namely, since the CCA in the FBE and LBE schemes corresponds to an operation of determining whether another communication device currently uses the unlicensed band prior to transmission, the CCA is performed at a transmission side.
FIG. 1B illustrates an example of a CCA operation of an LBE in an unlicensed band according to the related art.
Referring to FIG. 1B, the LBE performs transmission when the result obtained by performing CCA 111 shows that there is no communication device that currently uses the unlicensed band. However, when it is determined that another communication device currently uses the unlicensed band, the LBE may perform additional CCA, as opposed to the FBE. Hereinafter, the additional CCA is referred to as extended CCA (ECCA). The ECCA 113 is constituted by, for example, N CCAs 115, where N is an arbitrarily selected value between 1 and q, and q is a given value. The LBE performs transmission when the result obtained by performing the ECCA 113 shows that there is no communication device that currently uses the unlicensed band. In this case, the time 117 during which the LBE can occupy the unlicensed band is a maximum of (13/32)*q ms, and the LBE thereafter performs ECCA again to have an idle period 119 during that time.
An FBE and an LBE have the following advantages and shortcomings. First, the LBE will exhibit higher performance than the FBE in view of a probability to occupy an unlicensed band. The reason for this is that the FBE cannot perform CCA again for a fixed frame period if the FBE fails in CCA once, whereas the LBE can perform an operation of occupying an unlicensed band by performing ECCA, that is, N additional CCAs after the LBE fails in CCA. Next, the FBE is advantageous in that the FBE is simpler than the LBE in view of scheduling, for example, transmission of a physical downlink control channel (PDCCH) in a LTE system. The FBE can use an unlicensed band based on a sub-frame boundary (i.e., PDCCH transmission time point). However, the LBE cannot harmonize the time point when an unlicensed band starts to be used with a sub-frame boundary since the LBE arbitrarily selects N that is the number of times that CCA of ECCA is performed.
FIG. 42 illustrates an example of an LBE scheme using a reservation signal in an unlicensed band.
Referring to FIG. 42, in the example, the LBE may reserve a part of a first sub-frame (sub-frame #0) 4201 and may perform PDCCH and data transmission from a second sub-frame (sub-frame #1) 4203.
In addition, the FBE causes minor damage to the surrounding Wi-Fi that shares an unlicensed band, compared with the LBE. Generally, the LBE has a higher probability to occupy an unlicensed band than the FBE. This is because Wi-Fi is considered to deprive more opportunities to occupy the unlicensed band.
Accordingly, a method of maintaining access to a licensed band is required to provide a reliable cellular communication service in a moving environment even though a UE uses an unlicensed band. An available data transmission rate can be enhanced by transmitting data of a service sensitive to a delay (such as a voice, etc.) using a licensed band and by transmitting a data service that is insensitive to a delay selectively using an unlicensed band in addition to the licensed band.
A structure considered in order to use an unlicensed band in a cellular system broadly includes a carrier aggregation (CA) structure and a dual connectivity (DC) structure. The CA structure, which allows a primary cell (PCell) to operate in a licensed band and one or more SCells to operate in an unlicensed band, may allow initial access, arbitrary access, channel quality report, and ACK/NACK report for major control procedures to operate in the PCell to ensure performance. In contrast, the DC structure, in which a PUCCH SCell (PS Cell) having a PUCCH is configured separately from a PCell in an unlicensed band, may perform initial access, arbitrary access, channel quality report, and ACK/NACK report for major control procedures in the PSCell. In the present specification, the PCell may be replaced by an SCell for which a report resource is configured through a channel other than a PSCell or PUCCH.
The following procedure is required to determine the transmission capacity of a transmission/reception link in an existing cellular communication system that is the same as an LTE system. In a downlink, a UE measures a reference signal of an evolved node B (eNB) and reports the quality of the signal to the eNB. The reference signal of the eNB may include a common/cell-specific reference signal (CRS) given in common to all UEs in the region of the eNB, a dedicated reference signal (DRS) given only to a particular UE, a channel state information-RS (CSI-RS), and the like. The UE may be controlled by the eNB to periodically or aperiodically report channel quality to the eNB using a channel quality indicator (CQI). The UE uses an uplink control channel for the periodic report and uses an uplink data channel for the aperiodic report. Based on the CQI reported by the UE, the eNB performs a scheduling process to determine a UE to which a physical channel resource block is to be allocated, and informs of allocation information for each UE according to the result. The allocation information is notified as a control signal that is scrambled to cell-radio network temporary identity (C-RNTI) or multimedia broadcast multicast service (MBMS)-RNTI (M-RNTI) of the UE through a PDCCH, and the UE having received the allocation information receives an allocated physical channel resource block from a physical downlink shared channel (PDSCH) notified by the control signal. In an uplink, the eNB may measure a reference signal of the UE to identify the quality of the signal. The reference signal of the UE may use a sounding reference signal (SRS) periodically (2 to 320 ms) allocated to a particular UE by the eNB. Although differing from the current standard, the use of demodulation reference signal (DMRS) transmitted together with uplink data of the UE may also be considered for an operation in a shared band. Based on the CQI obtained by measuring the reference signal transmitted by the UE, the eNB performs a scheduling process to determine a UE to which a physical channel resource block is to be allocated, and informs of allocation information for each UE according to the result. The allocation information is notified as a control signal that is scrambled to C-RNTI or M-RNTI of the UE through a PDCCH, and the UE having received the allocation information transmits an allocated physical channel resource block from a physical uplink shared channel (PUSCH) notified by the control signal.
A predetermined delay time or more is spent due to signal transmission/reception and processing that are required by an eNB to complete channel measurement and link adaptation together with a UE. For example, in regard to an operation in a downlink, as in the example of FIG. 21.
FIG. 21 is a view for explaining a channel state information (CSI) feedback delay in a wireless communication system.
Referring to FIG. 21, two sub-frames are required by the UE to measure a reference signal 2101 transmitted by the eNB in every downlink sub-frame and to report a CQI 2103 to a physical uplink control channel (PUCCH) allocated to every uplink sub-frame. The eNB requires one sub-frame to perform channel estimation 2105 and one to k sub-frames 2107 according to implementations for scheduling to determine resource allocation and a modulation & coding scheme (MCS) index. Accordingly, an available minimum CQI feedback delay of 4 ms is required. Since the minimum period of a periodic CQI report resource is two sub-frames in an uplink, the uplink additionally has a delay corresponding to one sub-frame, compared with a downlink so that 5 ms is required. If the period of the CQI report resource increases, the entire CQI feedback delay also increases by the increment.
As described above, an LTE system may provide a minimal CQI feedback delay when measuring a periodic reference signal. However, the following three problems may happen in applying an existing link adaptation technique in a shared band, such as an unlicensed band, in which a rule for the coexistence between different communication systems is required.
A first problem is inaccuracy of periodic reference signal measurement. Due to the listen before talk (LBT) regulation, an eNB is not guaranteed to transmit a periodic reference signal, and even though there is no regulation, a measurement of the periodic reference signal is likely to vary seriously. Although a UE performs measurement in the position of a periodically allocated reference signal resource, if the UE does not know the fact that the eNB does not succeed in LBT, the measurement is withdrawn or is performed in a signal resource to which a reference signal is not actually transmitted. For example, even though there is an LBT regulation in a particular region, such as Europe, a short control signal (SCS) is exempted from using LBT. The condition for the SCS has to be designed such that a transmission device occupies and transmits, for example, only 5% of a resource within 50 ms. Even though a periodic reference signal can be transmitted in this way, the UE suffers from discontinuous interference from an adjacent Wi-Fi AP/UE or an LTE UE that belongs to an asynchronous cell eNB or an asynchronous cell. The discontinuous interference is caused by CCA or a hidden node. For example, if a device has a configured CCA threshold, a UE having received, from another eNB, an interference signal with power that is higher than the CCA threshold cannot perform transmission. In this case, whether devices adjacent to the eNB can perform transmission is determined according to the eNB's success or failure in LBT. Accordingly, a difference in the amount of interference may be noticeable. The first problem happens identically even when an uplink transmits an SRS of a UE.
A second problem is the use of a discontinuous wireless resource. For example, the problem may occur in a case of measuring an aperiodic reference signal. Namely, due to a problem in measurement of a periodic reference signal, a problem may happen even though an eNB measures a transmitted reference signal in a sub-frame in which data is transmitted after a success in LBT. Since a success or failure in LBT is arbitrary, the channel measured at the time point when LBT has succeeded last is more likely to differ from that measured at the current time point when LBT succeeds. Although the eNB may perform scheduling based on channel quality measured in the previous sub-frame only when a plurality of sub-frames are continuously allocated to one UE, a delay of at least 4 ms may occur as described above. Accordingly, a time point when resource allocation is performed based on a CQI measured in the n-th sub-frame is possible in the (n+4)-th sub-frame. While a resource has to be allocated to one UE beyond at least (n+4)-th sub-frame, it is impossible to ensure this since a large amount of traffic is not always stacked in a waiting queue. So, the eNB may minimize the number of physical resource blocks (PRBs) allocated to one UE in one sub-frame for continuous sub-frame allocation. However, since a dedicated RS (DRS, for a downlink) or demodulation RS (DMRS, for an uplink) included in an allocated data resource is used, it is impossible to identify the quality of a resource other than the PRB allocated by the eNB in the first sub-frame. That is, since there is no comparing channel qualities for various frequency PRBs, frequency selective scheduling is impossible only with an uplink signal.
A third problem is a delay of a CQI report time point. A delay time for CQI report occurs so that it may be difficult to accurately measure a channel. This may occur when data is transmitted to a downlink. When a UE having measured a reference signal transmitted by an eNB attempts to report with an uplink resource allocated by the eNB, a delay corresponding to one CCA period (for example, one sub-frame) may occur in an FBE scheme every time CCA fails. Since an LBE scheme has a problem in that multiple UEs within the service region of an eNB compete together, the LBE scheme is not preferred in an uplink.
The above information is presented as background information only, to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.